Tissue products are commonly produced by depositing cellulosic fibers suspended in water on a moving foraminous support to form a nascent web, removing water from the nascent web, adhering the dewatered web to a heated cylindrical Yankee dryer, and then removing the web from the Yankee with a creping blade which, in conventional processes, imparts crepe ridges extending generally transversely across the sheet, the machine direction, frequency of these crepe bars ranging from about 10 to about 150 crepe bars per inch of tissue. Tissues produced in this conventional fashion may often be considered lacking in bulk, appearance and softness and so require additional processing after creping, particularly when produced using conventional wet pressing technology. Tissues produced using the through air drying technique normally have sufficient bulk but may have an unattractive appearance. To overcome this, an overall pattern is imparted to the web during the forming and drying process by use of a patterned fabric having proprietary designs to enhance appearance that are not available to all producers. Further, through air dried tissues can be deficient in surface smoothness and softness unless strategies such as calendering, embossing and stratification of low coarseness fibers on the tissue""s outer layers are employed in addition to creping. Conventional tissues produced by wet pressing are almost universally subjected to various post-processing treatments after creping to impart softness and bulk. Commonly such tissues are subjected to various combinations of both calendering and embossing to bring the softness and bulk parameters into acceptable ranges for premium quality products. Calendering adversely affects bulk and may raise tensile modulus, which is inversely related to tissue softness. Embossing increases product caliper and can reduce modulus, but lowers strength and can hurt surface softness. Accordingly, it can be appreciated that these processes can have adverse effects on strength, appearance, surface smoothness and particularly thickness perception since there is a fundamental conflict between bulk and calendering.
The present invention is directed to tissue having highly desirable bulk, appearance and softness characteristics produced by a process utilizing a novel undulatory creping blade having a multiplicity of serrulations formed in its rake surface which presents differentiated creping angles and/or rake angles to the web as it is being creped. The invention is also directed to a novel blade having an undulatory rake surface having trough-shaped serrulations in the rake surface of the blade. The undulatory creping blade preferably has a multiplicity of alternating serrulated creping sections of either uniform depth or a multiplicity of arrays of serrulations having non-uniform undulatory depth. The present invention also relates to biaxially undulatory single-ply and multi-ply tissues, single-ply and multi-ply towels, single-ply and multi-ply napkins and other personal care and cleaning products as well as novel creping blades and the novel processes for producing such products.
Paper is generally manufactured by dispersing cellulosic fiber in an aqueous medium and then removing most of the liquid. The paper derives some of its structural integrity from the mechanical interlocking of the cellulosic fibers in the web, but most by far of the paper""s strength is derived from hydrogen bonding which links the cellulosic fibers to one another. With paper intended for use as bathroom tissue, the degree of strength imparted by this inter-fiber bonding, while necessary to the utility of the product, can result in a lack of perceived softness that is inimical to consumer acceptance. One common method of increasing the perceived softness and cushion of bathroom tissue is to crepe the paper. Creping is generally effected by fixing the cellulosic web to a Yankee drier with an adhesive/release agent combination and then scraping the web off the Yankee by means of a creping blade. Creping, by breaking a significant number of inter-fiber bonds adds to and increases the perceived softness of resulting bathroom tissue product. However, creping with a conventional blade alone may not be sufficient to impart the desired combinations of softness, bulk and appearance.
We have discovered that tissue having highly desirable bulk, appearance and softness characteristics, can be produced by a process similar to conventional processes, particularly conventional wet pressing, except that the conventional creping blade is replaced with an undulatory creping blade presenting differentiated creping and rake angles to the sheet and having a multiplicity of spaced serrulated creping sections of either uniform depths or non-uniform arrays of depths. The depths of the undulations are above about 0.008 inches.
Techniques for creping of tissue and towel weight papers using patterned or non-uniform creping blades are known but these known techniques rather than being suitable for production of premium quality bath tissue, facial tissue or kitchen toweling, have been suggested for, and seem more suited for, production of wadding or insulating papers or other extremely coarse papers.
Three references of interest are Fuerst, U.S. Pat. No. 3,507,745; B.D. Nobbe, U.S. Pat. No. 3,163,575; and possibly British Pat. No. 456,032. Fuerst, U.S. Pat. No. 3,507,745, suggests use of a highly beveled blade which has square shouldered notches formed into the rake surface. This type of a blade is said to be suitable for producing very high bulk for cushioning and insulation purposes but, in our opinion, is not suitable for premium quality towel and tissue products. The depth of the Fuerst blades"" notches are only about 0.0015 inches to 0.007 inches.
Nobbe, U.S. Pat. No. 3,163,575, describes a doctor blade for differentially creping sheets from a drum to produce a product which is quite similar to that of the Fuerst patent. The Nobbe patent describes a blade with a relatively flat bevel angle into which notches have been cut, defining regions having a very large bevel angle. The crepe in the portions of the sheet that contact the notched portions of the blade will have quite a coarse crepe or no crepe, while the areas of the sheet that contact the unnotched blade portions will have a fine crepe.
In the Fuerst patent, the unmodified blade has a very large bevel angle, with portions of its creping edge being flattened to produce a surface that results in fine crepe in the portion of the sheet that contact this surface. The portions of the sheet that contact the unmodified sections of the blade will have very coarse crepe, thus giving an appearance of having almost no crepe. Our experience suggests that neither the Nobbe nor the Fuerst blades are suitable for the manufacture of commercially acceptable premium quality tissue and towel products.
Pashley, British Patent No. 456,032, teaches creping of a sheet from a drum using a creping blade whose edge has been, serrated in a sawtooth pattern, the teeth being about one-eight (0.125) inch deep and numbering about 8 to the inch. The distance from tip to base of these teeth is about 2 to about 25 times the depth of the undulations that are cut into the present crepe blade. The product described in the Pashley patent has crepe that is much coarser and more irregular than the crepe of a product made using conventional creping technology. While this type of product may hold some advantages in the manufacture of crepe wadding, a product having such a coarse crepe would not normally be considered acceptable for use in premium tissue and towel products.
What has been needed is a simple, reliable process for creping tissue weight substrates to produce desirable products having higher caliper at lower basis weight than are produced in processes using a conventional creping blade. Products made using the creping procedure of the present invention will have a crepe fineness similar to that of conventionally-made tissue sheets but the resulting web combines crepe bars extending in the cross direction with undulations extending in the machine direction.
We have discovered that tissue having highly desirable bulk, appearance and softness characteristics, can be produced by a process similar to conventional processes, particularly conventional wet pressing, by replacing the conventional creping blade with an undulatory creping blade having a multiplicity of serrulated creping sections presenting differentiated creping and rake angles to the sheet. The depth of the undulations is preferably above about 0.008 inches, more preferably between about 0.010 inches and about 0.040 inches. Further, in addition to imparting desirable initial characteristics directly to the sheet, the process of the present invention produces a sheet which is more capable of withstanding calendering without excessive degradation than a conventional wet press tissue web. Accordingly, using this creping technique it is possible to achieve overall processes which are more forgiving and flexible than conventional existing processes. In particular, the overall processes can be used to provide not only desirable premium products including high softness tissues and towels having surprisingly high strength accompanied by high bulk and absorbency, but also to provide surprising combinations of bulk, strength and absorbency which are desirable for lower grade commercial products. For example, in commercial (away-from-home) toweling, it is usually considered important to put quite a long length of toweling on a relatively small diameter roll. In the past, this has severely restricted the absorbency of these commercial toweling products as absorbency suffered severely from the processing used to produce toweling having limited bulk, or more precisely, the processing used to increase absorbency also increased bulk to a degree which was detrimental to the intended application. The process of the present invention makes it possible to achieve surprisingly high absorbency in a relatively non-bulky towel thus providing an important new benefit to this market segment. Similarly, many webs of the present invention can be calendered more heavily than many conventional webs while still retaining bulk and absorbency, making it possible to provide smoother, and thereby softer feeling, surfaces without unduly increasing tensile modulus or unduly degrading bulk. On the other hand, if the primary goal is to save on the cost of raw materials, the tissue of the present invention can have surprising bulk at a low basis weight without an excessive sacrifice in strength or at low percent crepe while maintaining high caliper. Accordingly, it can be appreciated that the advantages of the present invention can be manipulated to produce novel products having many combinations of properties which previously were somewhat impractical.
Further, it appears that the process producing these advantages is at least comparable in runnability and forgivingness to conventional creping processes and may be run on equipment adapted to use conventional creping blades as the undulatory creping blades of the present invention will fit into conventional holders and will operate at roughly equivalent holder angles. The life of the preferred undulatory blades seems to be at least about the same as the life expected with conventional blades. At this time, preliminary results indicate that the life of preferred undulatory creping blades according to the present invention could possibly even be significantly greater than the life of a conventional blade, although, to be able to claim this definitively would require a substantial amount of commercial operating data which are, of course, simply not available. Preliminary data also indicate that care must be taken in operating the undulatory creping blade to collect dust formed.
In contrast to conventional tissues having creping bars generally running transversely, the tissue of the present invention has a biaxially undulatory surface wherein the transversely extending crepe bars are intersected by longitudinally extending undulations imparted by the undulatory creping blade.